dump HWC state in dumpsys

Change-Id: Ifbb38ca1ac9685776250e01c8d02021b35af72cf
diff --git a/services/surfaceflinger/DisplayHardware/HWComposer.h b/services/surfaceflinger/DisplayHardware/HWComposer.h
index 22ff10c..5a9e9eb 100644
--- a/services/surfaceflinger/DisplayHardware/HWComposer.h
+++ b/services/surfaceflinger/DisplayHardware/HWComposer.h
@@ -27,6 +27,8 @@
 namespace android {
 // ---------------------------------------------------------------------------
 
+class String8;
+
 class HWComposer
 {
 public:
@@ -54,6 +56,9 @@
     size_t getNumLayers() const;
     hwc_layer_t* getLayers() const;
 
+    // for debugging
+    void dump(String8& out, char* scratch, size_t SIZE) const;
+
 private:
     hw_module_t const*      mModule;
     hwc_composer_device_t*  mHwc;